The Stovebolt Site Forums The Shop Area The HiPo Shop 274 cubic inch stovebolt- - - - -

Since the GMC 270 engines are getting scarce, and rebuild parts are either expensive or unobtanium, here's a way to get 270+ cubic inches from a Chevy 261, with the same stroke (and torque) of the GMC engine. The secret is in an oddball high performance connecting rod made for big block Chevy engines. It's an H-beam rod that's 7.1 inches long, just a bit longer than the GMC 7 inch rod. By offset-grinding the 235/261 rod journal a few thousandths the 3 15/16" stroke can be changed to 4 inches, and the rod journal can be ground down to 2.200 "- - - -the same as a big block Chevy rod bearing. By using a .060" oversize Chevy 265 V8 piston, the resulting bore/stroke/connecting rod setup produces 274 cubic inches, and the piston crown is .005" proud of the block. If you want zero deck height, just take a skim cut off the top of the piston with a lathe. A Ford 292 V8 piston, also 60 over, puts the piston crown .015" below deck height. Since the big block rod is thinner than the stovebolt rod, use shims at the top of the rod over the wrist pin to keep the rod centered in the piston.

The same stroke and connecting rod setup applied to a 235, using standard bore pistons for a 1955-64 Ford 223 six gives 248 cubic inches and puts the piston about .018" below the top of the block. Either engine, with the 848 cylinder head, will get a noticeable boost in compression from the increased displacement.

The advantage to the above modifications is that modern, more available "over the counter" parts can be used, with a minimum of fancy machining or buying spendy custom made parts!
Jerry

Jerry:

Two questions for you.

1. How much for you to build and ship one of those to Newport, RI?
2. How fast can you have it done?

I'm watching for the UPS truck!

😉

Larry

LOL! I'm still in the "dreams and schemes" stages, but I'll need to get someone to road test a prototype one of these days. I also need to get my water brake dyno back into service, mount it on a trailer, and hit a few car shows and stovebolt roundups to do a "Show and Tell" demonstration. I have a feeling something like that would keep my son and grandson busy building up engines for quite a while! There's also a 258 cubic inch "216" on the drawing board with full pressure lubrication to the connecting rods that looks just like an original engine from the outside!
Jerry

I think one of those would be the perfect motor for my Suburban. The transplant 250 in there now is getting pretty tired. I need a little more oomph!

I am game. If you will walk me through some of the additional details, I will build one. I do not have any spare 261 blocks left but I have a couple of 235 motors. Maybe start with the rods.

The block deck height on the 235 and the 261 is 10.875 inches. There are two lengths of big block Chevy high performance rods available, one with a 6.8" length and one that is 7.1" long. Both of them have a 2.2" rod journal size, with the usual 10, 20, and 30 undersizes available. The stovebolt crankshaft can be reground to the 2.2" diameter, and offset to give a 4" stroke instead of the 3 15/16 of the original 235/261. A 235 piston can be used by putting a bushing in the big block rod to match the 235 wrist pin size. Use aluminum or Teflon buttons in the ends of the wrist pin to keep the pin centered in the piston. Since the BBC rod is narrower than the stovebolt rod, use brass spacer washers on both sides of the rod around the wrist pin to keep the rod centered on the crankshaft journal. Here's how the tolerances stack up:

BBC rod = 6.8"
1/2 stroke = 2"
235 piston compression height = 2.06"
Total rod/stroke/compression height = 10.86"
Stovebolt block height = 10.875
Piston is .015" below the top of the block.

4" stroke X .060" oversize 235 piston = 247.2 cubic inches.

Even an undersize stovebolt crankshaft can be reground to the 4" stroke by using an undersize BBC rod bearing. The narrower, smaller diameter rod bearing will reduce the parasitic drag on the rotating assembly.

www.ebay.com/itm/174340044115?

There is also a way to use 7" long 1949-53 flathead Ford V8 connecting rods and Ford 223 cubic inch inline six pistons in a 235 to get a longer stroke and more displacement. (4 1/8" stroke, 255 CI) That combination puts the piston .044" above the top of the block, which can be machined for zero deck height without seriously compromising the strength of the piston.

Jerry